: A series of studies are proposed that will analyze how auditory cortex, the medial geniculate body, and the inferior colliculus are interrelated anatomically. The primary purpose is to use connectional, neurochemical, and structural data to understand the basic circuitry involved in the analysis of auditory information in the midbrain, thalamus, and cerebral cortex. The immediate goals are to describe ipsilateral and contralateral cortical connections, which play a role in higher-order processing; to analyze thalamocortical connectivity, which may mediate parallel processing by different cortical subdivisions; to explore corticothalamic projections that could modulate ascending information transmission, and to examine corticopontine input for possible premotor control. These data can demonstrate the substrates for interactions between the cortex and the thalamus, and between primary and nonprimary auditory cortex. If there is a hierarchical arrangement among auditory cortical areas, these experiments should define the arrangement of feedforward connections. Neurochemical studies in the inferior colliculus will examine the distribution of putative inhibitory neurotransmitters, and whether a neural subpopulation is also positive for nitric oxide synthase, a neuromodulator. The types and concentration of local circuit neurons in the midbrain will be compared in primary and secondary subdivisions of the inferior colliculus. Such data can demonstrate differential inhibitory substrates essential for normal hearing and those dedicated to the analysis of sound locus in space or the neural control of vocalizations. Other experiments in cat and monkey will combine physiological mapping in auditory cortex with tracer deposits to demonstrate the connectivity of functionally defined cortical subregions. These studies will show whether thalamocortical and corticocortical connections are organized topographically and, if so, the dimension that defines their organization, for example, sharpness of tuning, aurality, amplitopy, or some other measure. A fourth approach is comparative. Projections from the inferior colliculus to the medial geniculate body will be examined in the rat and related to results from the cat; a neurochernical analysis of the bat auditory cortex will be compared to patterns in the cat to determine whether more than one pattern of inhibitory organization exists in the same brain parts in mammals. Such data could shed light on the evolution of inhibitory circuitry and their phenotypical patterns. The connectional, neurochemical, functional, and comparative approaches provide complementary perspectives on the design and function of these neural systems and their interactions.